A nonaqueous electrolyte battery, particularly, a lithium secondary battery using a carbon material or lithium titanium oxide as a negative electrode active material and a layered oxide containing nickel, cobalt, and manganese as positive electrode active materials has already been put into practical use as a power supply in a broad field. The form of such a nonaqueous electrolyte battery widely ranges from a small battery for various kinds of electronic devices to a large battery for an electric automobile. These lithium secondary batteries use, as the electrolytic solution, a nonaqueous organic solvent containing ethylene carbonate or methyl ethyl carbonate, unlike a nickel hydrogen battery or a lead storage battery. Electrolytic solutions using these solvents have high resistance to oxidation and high resistance to reduction as compared to an aqueous electrolyte solution, and electrolysis of the solvents hardly occurs. For this reason, a nonaqueous lithium secondary battery can implement a high electromotive force of 2 to 4.5 V.
On the other hand, since many organic solvents are combustible, the safety of a secondary battery using an organic solvent readily becomes lower than that of a secondary battery using an aqueous solution in principle. Although various measures are taken to improve the safety of a lithium secondary battery using an electrolytic solution containing an organic solvent, they are not necessarily enough. In addition, a nonaqueous lithium secondary battery requires a dry environment in its manufacturing process, and the manufacturing cost inevitably rises. Furthermore, since an electrolytic solution containing an organic solvent is poor in conductivity, the internal resistance of the nonaqueous lithium secondary battery readily becomes high. These problems are great disadvantages for a large storage battery used in an electronic automobile, a hybrid electronic automobile, or an electric power storage for which the battery safety and the battery cost are of importance.
To solve these problems, forming an electrolytic solution as an aqueous solution has been examined. In an aqueous electrolytic solution, a potential range in which charge and discharge of a battery are executed needs to be limited to a potential range in which an electrolysis reaction of water contained as a solvent does not occur. For example, when lithium manganese oxide is used as a positive electrode active material, and lithium vanadium oxide is used as a negative electrode active material, electrolysis of water can be avoided. With the combination of these materials, an electromotive force of about 1 to 1.5 V is obtained. However, a sufficient energy density as a battery can hardly be obtained.
In addition, when lithium manganese oxide is used as a positive electrode active material, and lithium titanium oxide is used as a negative electrode active material, an electromotive force of about 2.7 V is obtained theoretically, and the battery can be attractive from the viewpoint of energy density as well. Such a combination of a positive electrode active material and a negative electrode active material can provide a high life characteristic when an organic electrolytic solution is used, and has already been put into practical use. However, when an aqueous electrolytic solution is used, the potential of lithium insertion/extraction of lithium titanium oxide is about 1.5 V (vs. Li/Li+). Hence, there is a problem that electrolysis of the aqueous electrolytic solution readily occurs. On the other hand, even in lithium manganese oxide of the positive electrode, a gas is generated due to oxidation of cations in the aqueous solution, and satisfactory charge is impossible.